36 research outputs found
Photo-induced modification and relaxation dynamics of Weyl-semimetals
The use of ultrashort laser pulses to investigate the response of materials
on femtosecond time-scales enables detailed tracking of charge, spin and
lattice degrees of freedom. When pushing the limits of the experimental
resolution, connection to theoretical modeling becomes increasingly important
in order to infer causality relations. Weyl-semimetals is particular class of
materials of recent focus due to the topological protection of the Weyl-nodes,
resulting in a number of fundamentally interesting phenomena. In this work, we
provide a first-principles framework based on time-dependent density-functional
theory for tracking the distribution of Weyl-nodes in the Brillouin-zone
following an excitation by a laser pulse. For the material TaAs, we show that
residual shifts in the Weyl-Nodes' position and energy distribution is induced
by a photo-excitation within femto-seconds, even when the laser-frequency is
off-resonant with the Weyl-node. Further, we provide information about the
relaxation pathway of the photoexcited bands through lattice vibrations
Itinerant magnetic multipole moments of rank five, triakontadipoles, as the hidden order in URuSi
A broken symmetry ground state without any magnetic moments has been
calculated by means of local-density-approximation to density functional theory
plus a local exchange term, the so-called LDA+ approach, for
URuSi. The solution is analysed in terms of a multipole tensor
expansion of the itinerant density matrix and is found to be a non-trivial
magnetic multipole. Analysis and further calculations show that this type of
multipole enters naturally in time reversal breaking in presence of large
effective spin-orbit coupling and co-exists with magnetic moments for most
magnetic actinidesComment: 5 pages, 3 figure
Multipole decomposition of LDA+ energy and its application to actinides compounds
A general reformulation of the exchange energy of -shell is applied in
the analysis of the magnetic structure of various actinides compounds in the
framework of LDA+U method. The calculations are performed in an efficient
scheme with essentially only one free parameter, the screening length. The
results are analysed in terms of different polarisation channels, due to
different multipoles. Generally it is found that the spin-orbital polarisation
is dominating. This can be viewed as a strong enhancement of the spin-orbit
coupling in these systems. This leads to a drastic decrease in spin
polarisation, in accordance with experiments. The calculations are able to
correctly differentiate magnetic and non-magnetic Pu system. Finally, in all
magnetic systems a new multipolar order is observed, whose polarisation energy
is often larger in magnitude than that of spin polarisation.Comment: Fixed some references and picture
A route towards finding large magnetic anisotropy in nano-composites: application to a WRe/Fe multilayer
We suggest here a novel nano-laminate, 5[Fe]/2[WRe] (x=0.6-0.8),
with enhanced magnetic hardness in combination with a large saturation moment.
The calculated magnetic anisotropy of this material reaches values of 5.3-7.0
MJ/m, depending on alloying conditions. We also propose a recipe in how to
identify other novel magnetic materials, such as nano-laminates and
multilayers, with large magnetic anisotropy in combination with a high
saturation moment
Optically controlling the competition between spin flips and intersite spin transfer in a Heusler half-metal on sub-100 fs timescales
The direct manipulation of spins via light may provide a path toward
ultrafast energy-efficient devices. However, distinguishing the microscopic
processes that can occur during ultrafast laser excitation in magnetic alloys
is challenging. Here, we study the Heusler compound Co2MnGa, a material that
exhibits very strong light-induced spin transfers across the entire M-edge. By
combining the element-specificity of extreme ultraviolet high harmonic probes
with time-dependent density functional theory, we disentangle the competition
between three ultrafast light-induced processes that occur in Co2MnGa:
same-site Co-Co spin transfer, intersite Co-Mn spin transfer, and ultrafast
spin-flips mediated by spin-orbit coupling. By measuring the dynamic magnetic
asymmetry across the entire M-edges of the two magnetic sublattices involved,
we uncover the relative dominance of these processes at different probe energy
regions and times during the laser pulse. Our combined approach enables a
comprehensive microscopic interpretation of laser-induced magnetization
dynamics on timescales shorter than 100 fs.Comment: 31 pages, 12 figure
Theoretical Studies of Magnetism and Electron Correlation in Solids
This work presents new development and applications of ab-initio simulation tools for material science. Focus lies on materials with strong electronic correlation and strong spin-orbit coupling. Improvements on methods for solving the impurity problem in LDA+DMFT is presented, as well as a reliant method for charge self-consistency in a LMTO based electronic structure code. A new adaptive scheme for Brillouin zone integration is developed, where we show a strong reduction of numerical noise compared to standard techniques. A reformulation of the standard LDA+U method aiming to reduce the number of free parameters is introduced. Fast and realistic reduction of the number of free parameters provides the possibility of high throughput calculations and enabled us to study a large number of compounds. An analysis method for polarization in terms of coupled multipoles, and their corresponding energy contributions is developed and applied. This led to the formulation of Katt's rules, a set of rules complementary to Hund's rules. Katt's rules applies for occupying the orbitals of an electronic shell with strong spin-orbit coupling. The analysis is also used to investigate the unconventional Uranium based superconductors URu2Si2, UPt3, UPd2Al3 and UNi2Al3, as well as the high temperature superconductor LaOFeAs. We also investigate the non-magnetic delta-phase of Plutonium, providing insight to the electronic structure and the branching ratios of 4d to 5f transitions seen in photo emission spectra.The influence of surface reconstruction on the magneto crystalline anisotropy is investigated in multilayer Fe/ZnSe, showing that Fe deposited on an unreconstructed interface strongly reduces the uniaxial component of the MAE. We provide a detailed understanding of the magnetic properties of Fe2P, opening possible routes for enhancing the MAE in this system. A general route to strong MAE in nano-laminates is presented, we apply this to propose a candidate with extremely strong anisotropy energy density, 5Fe/2W1-xReX for x=[0.6-0.8]
Theoretical Studies of Magnetism and Electron Correlation in Solids
This work presents new development and applications of ab-initio simulation tools for material science. Focus lies on materials with strong electronic correlation and strong spin-orbit coupling. Improvements on methods for solving the impurity problem in LDA+DMFT is presented, as well as a reliant method for charge self-consistency in a LMTO based electronic structure code. A new adaptive scheme for Brillouin zone integration is developed, where we show a strong reduction of numerical noise compared to standard techniques. A reformulation of the standard LDA+U method aiming to reduce the number of free parameters is introduced. Fast and realistic reduction of the number of free parameters provides the possibility of high throughput calculations and enabled us to study a large number of compounds. An analysis method for polarization in terms of coupled multipoles, and their corresponding energy contributions is developed and applied. This led to the formulation of Katt's rules, a set of rules complementary to Hund's rules. Katt's rules applies for occupying the orbitals of an electronic shell with strong spin-orbit coupling. The analysis is also used to investigate the unconventional Uranium based superconductors URu2Si2, UPt3, UPd2Al3 and UNi2Al3, as well as the high temperature superconductor LaOFeAs. We also investigate the non-magnetic delta-phase of Plutonium, providing insight to the electronic structure and the branching ratios of 4d to 5f transitions seen in photo emission spectra.The influence of surface reconstruction on the magneto crystalline anisotropy is investigated in multilayer Fe/ZnSe, showing that Fe deposited on an unreconstructed interface strongly reduces the uniaxial component of the MAE. We provide a detailed understanding of the magnetic properties of Fe2P, opening possible routes for enhancing the MAE in this system. A general route to strong MAE in nano-laminates is presented, we apply this to propose a candidate with extremely strong anisotropy energy density, 5Fe/2W1-xReX for x=[0.6-0.8]